Moderna and Merck shares fall after melanoma vaccine data appears to have leaked and is significantly below expectations.

Reports say that the highly anticipated personalized melanoma vaccine results from the two companies have leaked and the treatment effect fell short of expectations, sending Moderna and Merck shares lower. The two companies announced in August that the customized vaccine, combined with Merck's blockbuster drug Keytruda, achieved a "significant and clinically meaningful" effect in reducing recurrence of this deadly tumor. Moderna's stock surged sharply when the news was announced, but specific data were not disclosed at the time. Detailed results are scheduled to be officially presented on Saturday at the European Society for Medical Oncology annual meeting in Madrid. On Thursday, details about the vaccine began to leak from unverified sources on social platforms, and Moderna and Merck shares moved lower. One account posted a report attributed to Oncology Central saying the risk of recurrence or death was reduced by 42%. However, that figure falls short of the benchmark in investors' memory. An earlier study involving a different patient population and a different time period showed the combination reduced the risk of recurrence or death by 49% and the risk of distant metastasis or death by 59%. The leaked figure of 42% is clearly below 49%, while key metrics from the Phase III trialincluding the full hazard ratio, confidence intervals and event countsremain undisclosed. In the absence of formal data, the market chose to price in the "known lower number."